Differential host utilisation by different life history stages of the fish ectoparasite Argulus foliaceus (Crustacea: Branchiura).

In this study we examine differences in the occurrence of life history stages of the destructive fish ectoparasite Argulus foliaceus (L., 1758) on eight fish species (stickleback, rudd, roach, gudgeon, bream, tench, crucian carp and common carp) sampled from a mixed-species recreational fishing lake on nine occasions during late spring and summer. Total numbers ofA. foliaceus, as well as the number of larval, juvenile and adult parasite stages, from each fish were recorded along with the fish species. Lice generally exhibited an aggregated distribution approximating a negative binomial distribution. Significant differences in the prevalence, intensity and intensity frequency distribution were observed between life history stages and between host species. In general, all life history stages of A. foliaceus exhibited an over-dispersed distribution. However, larval lice did show some degree of aggregation particularly within the stickleback samples. Infection data for parasite larval stages suggested that sticklebacks are more likely to be infected than other host species. For adult lice, however, carp appeared to be the main host. We propose that A. foliaceus infection characteristics are predominantly determined by the level of host exposure to the parasite and its life history stages (larval, juvenile and adult) rather than by an innate difference in host susceptibility related to individual host factors such as immune responses. We conclude that host exposure is determined by the parasite-host behavioural interplay related to species-specific ecology and behavioural traits such as microhabitat preference and normal swimming speed.

Environment affects stress in exercised turbot.

We investigated the interaction of water temperature (10, 18 and 22 degrees C) and salinity (33.5 and 15 per thousand ) on the stress response of juvenile turbot. At each temperature/salinity combination, fish were subjected to 10 min enforced exercise. This induced a moderate stress response, which differed at the various temperature and salinity combinations. High temperatures caused more rapid increases in plasma cortisol and glucose, larger and more rapid increases in plasma lactate levels, which were also influenced by body weight, and a faster recovery in plasma Na(+) and Cl(-). Low salinity ameliorated cortisol responses at low but not at high temperatures. The magnitude of ionic disturbance was reduced at 15 per thousand. Plasma K(+) did not change at any temperature or salinity. The stress response involved activation of the brain-pituitary-interrenal axis, as indicated by the cortisol elevations. The low magnitude of glucose responses, the mild Na(+) and Cl(-) disturbances, and the lacking K(+)-responses indicated mild activation of the brain-sympathetic-chromaffin cell axis, and hence a low release of catecholamines, which seemed though to occur to a higher extent at higher temperatures. The relatively low catecholaminergic response of turbot may be linked to their inactive sedentary lifestyle. The higher responsiveness at higher water temperatures may reflect a higher overall adaptive capacity.

Regulation of branchial Na(+)/K(+)-ATPase in common carp Cyprinus carpio L. acclimated to different temperatures.

Isogenic carp Cyprinus carpio L. were acclimated to water temperatures of 15, 22 and 29 degrees C for at least 8 weeks. The acclimations consistently resulted in slightly, but significantly, different plasma osmolality, sodium, potassium and chloride concentrations between the groups studied. Plasma total and ionic calcium levels were unaffected, indicating successful adaptation. The apparent changes in set point for plasma ion levels are explained by altered sodium pump activity and hormonal control of branchial permeability to water and ions. It appears that in 15 degrees C-acclimated fish, a lower apparent Na(+)/K(+)-ATPase activity is compensated by strongly enhanced Na(+)/K(+)-ATPase expression (determined biochemically and immunohistochemically). In 29 degrees C-acclimated fish, the higher ambient temperature activates the enzyme. Arrhenius plots for branchial Na(+)/K(+)-ATPase preparations of the three groups of fish suggest the occurrence of different enzyme isoforms or protein (in)stability as explanations for differences in apparent enzyme activities, rather than temperature-dependent changes in membrane fluidity. As for hormonal control over permeability, prolactin mRNA expression (and anticipated production and release) is lower in fish kept at 29 degrees C, suggesting that control over branchial permeability to water and ions needs to be downregulated at higher temperatures. In so doing, enhanced sodium pump activity is balanced by a controlled passive ion loss to fine-tune plasma sodium levels. Basal plasma cortisol levels did not correlate positively with Na(+)/K(+)-ATPase expression, but doubling plasma cortisol levels in control fish by administering exogenous cortisol (for 7 days, using implanted minipumps and thus stress-free) enhanced Na(+)/K(+)-ATPase expression. This effect must be the result of a glucocorticoid action of the steroid: in fish, mineralocorticoid receptors have higher affinity for cortisol than glucocorticoid receptors. At a lower ambient temperature, branchial Na(+)/K(+)-ATPase expression is upregulated to counteract the temperature-inhibited activity of the sodium pump, perhaps via a mineralocorticoid receptor.